1. Technical Field
The present invention relates to an industrial robot, and more particularly to a horizontal articulated robot having a plurality of horizontally rotating arms.
2. Related Art
Among industrial robots, there is known a SCARA robot (a horizontal articulated robot) with a plurality of arms sequentially connected by means of a horizontal articulation. FIG. 12 shows an example of a planar structure of a common SCARA robot having two arms.
As shown in FIG. 12, the SCARA robot includes a base 51, a first arm 52 having a base end portion horizontally rotatably connected to a base 51, and a second arm 53 having a base end portion connected to an extreme end portion of the first arm 52 by a horizontal articulation.
FIG. 13 shows an outline of a movable range in the SCARA robot thus structured. As in the drawing, the first and the second arms 52 and 53 work together to move a working portion 53a disposed at an extreme end portion of the second arm 53 to an arbitrary position in a movable range WA formed between a maximum radius Rmax and a minimum radius Rmin. In other words, by using the working portion 53a at the extreme end portion, the SCARA robot can perform various operations for a target object (such as an article to be processed) located in the arbitrary position in the movable range WA.
Meanwhile, in the SCARA robot as above, the maximum radius Rmax defining the movable range WA is determined based on a maximum rotation radius Dmax obtained by adding an arm length L11 equivalent to a distance between a base rotation axis C11 of the first arm 52 and a connection rotation axis C12 of the second arm 53 to an arm length L12 equivalent to a distance between the connection rotation axis C12 and a working shaft C13 of the second arm 53. Similarly, the minimum radius Rmin defining the movable range WA is determined based on a minimum rotation radius Dmin equivalent to a distance between the base rotation axis C11 and a position Pa or Pb (See FIG. 12) where the working portion 53a can come closest to the base 51 without interfering with the first arm 52. In the arm structure thus provided, basically, the movable range WA is made largest when the minimum radius Rmin is set so as to be equivalent to a difference between the arm lengths L11 and L12. However, an arm frame of the first arm 52 is required to be large enough to maintain strength and rigidity for supporting the second arm 53 and others. Accordingly, miniaturization of the arm frame is restricted, and it is thus difficult to reduce the minimum radius Rmin.
Thus, in order to extend the movable range of such a SCARA robot, for example, there is proposed a conventional SCARA robot as in JP-A-2007-168004. The proposed SCARA robot includes a first arm horizontally rotatably mounted on a base and a second arm connected to the first arm by a horizontal articulation. The second arm is formed to be longer than the first arm. In other words, with respect to a maximum rotation radius of a working portion at an extreme end portion of the second arm obtained when both arms are linearly expanded, a minimum rotation radius of the working portion located in a position closest to the base by collapsing the second arm is reduced to increase the movable range in the SCARA robot.
On the other hand, the SCARA robot as above is generally expected not only to have as large a movable range as possible but also to have a more compact size, a high responsiveness, a high positioning precision, and the like.
In the SCARA robot proposed as above, however, although the movable range is surely extended, the second arm longer than the first arm naturally becomes larger in mass and inertia. This can deteriorate responsiveness upon movement and positioning precision.